The present invention relates to apparatus for shifting the phase of quadrature video signals and, more particularly, to novel video phase-shift apparatus for use in the transmitter and receiver of a nuclear magnetic resonance (NMR) imaging system.
Apparatus for providing a pair of video signals (i.e. a pair of signals having relatively low-frequency, including DC, components; sometimes referred to as "baseband" signals) with one of a plurality of discrete phases, relative to a reference phase, is often required in the medical imaging arts. One use of such apparatus is in the transmitter and receiver portions of an RF spectrometer utilized, as a part of a nuclear magnetic resonance spectrometry and imaging system, for providing a selected one of a plurality of distinct phase shifts. A number of discrete phase shifts may be needed when acquiring data from a sample in: zero-quantum and/or multiple-quantum experiments; increasing suppression of imaging artifacts; dual channel excitation control; and the like. Thus, many high-resolution NMR imaging procedures may require manipulation of the phase of either, or both, of (a) a pair of sample excitation baseband signals, determining the modulation envelope of a signal from the NMR system transmitter, and (b) a pair of sample response baseband signals, appearing as the recovered video envelope of the signal received from the sample. Therefore, it is desirable to provide precise phase control for any NMR spectrometer, and particularly in a spectrometer used for research purposes. As a minimum, the ability to shift the relative phase of each baseband video analog signal pair by a selected one of a 0.degree. , 90.degree. , 180.degree. or 270.degree. increment is required. Such phase-shifting should be performed under control of the system computer and must be independent of each excitation pulse signal (typically in both a decoupler channel and an observer channel), as each signal must have the capability to be set with its own phase and timing characteristics. While some of the desired characteristics might be achievable by radio-frequency (RF) phase-shifting of either, or both, of the excitation RF signal (after amplitude modulation in the transmitter) and the response RF signal (prior to demodulation in the receiver), it is desirable to provide one of four quadrants of phase shift to the "audio" baseband signals modulating the RF excitation signal and to the "audio" baseband signals recovered from the response signal, to assure that desired procedures (such as multiple-quantum experiments) can be carried out.